The disposal of waste rubber tires is major concern throughout the world. The U.S. alone adds approximately 270 million waste tires (3.6 million tons) annually to the several billions that already overflow landfills and causes environmental and health hazards. Many jurisdictions impose license fees or special taxes to defray the costs associated with tire disposal.
Recycling waste tire in the form of the crumb rubber in concrete has been developed as a cost effective solution to this problem. Rubberized concrete, which is less dense but more ductile than conventional concrete, is tough and impact resistance and is used, for instance, in noise barriers, heat insulation, and vibration dampening. Unfortunately, as crumb rubber replaces some of the aggregate in concrete, the compressive strength and splitting tensile strength of the resulting rubberized concrete are reduced proportionally. This severely limits the amount of crumb rubber that can be incorporated and curtails the range of rubberized concrete's applications. U.S. Pat. No. 5,456,751 to Zandi et al. discloses concrete compositions containing 0.05 to 20 percent by weight particulate rubber that are derived from recycled waste tires. The addition of rubber reduces the compressive strength of the concrete from 4,000 psi to 1,900 psi. This reduction occurs even in the presence of a “superplasticizer” that enhances the adhesion of tire rubber particles to the cement paste, which has been a major problem of rubberized concrete. U.S. Pat. No. 5,762,702 to Guy describes a rubberized concrete composition containing about 3.5 to 4.0 percent by weight shredded particulates comprising rubber, fibers and steel, that are obtained from whole waste tires, fly ash additives and a superplasticizer. Test cylinders made from the concrete composite exhibit a compressive strength of 4,000 psi at 7 days. The patent however provides no test data.
U.S. Patent Application 20050096412 to Petr et al. describes concrete compositions containing rubber aggregates, having distinct geometric shapes, along with superplasticizers, fly ash, carbon fiber, fiberglass, and steel that are mixed with the concrete compositions to vary properties of the finish products. The compressive strength of the rubberized concrete is lower than that of conventional concrete.
The primary reason for the lowered compressive and tensile strengths of rubberized concrete is that the crumb rubber particulates form relatively weak physical bonds with the concrete matrix. Superplasticizers appear to enhance the physical bonding between the hydrophobic rubber particulates and the hydrophilic concrete matrix that results in a moderately stronger rubberized concrete composite moderately.
Various surface techniques have been developed to promote stronger bonding between the rubber particles and the surrounding cement matrix. These techniques require, for instance, washing rubber particles with water, modifying the rubber surface with acid etching or plasma treatment, and employing coupling agents. Sodium hydroxide (NaOH) treatment appears to achieve the best results in enhancing the adhesion of tire rubber particles to the cement paste although the resulting rubberized concrete still show a 33 percent reduction in compression strength as compared to the conventional concrete. See, Segre, N. and Joekes, I., “Use of tire rubber particles as addition to cement paste”, Cement and Concrete Research 2000, 30 (9), 1421-1425.
Chou et al. proposed a theoretical analysis to explain the effect of rubber additives on mechanical properties of the rubberized concrete and showed that the addition of rubber particles could block water difficusion in rubberized concrete, leading to insufficient and imperfect hydration in some regions, even when the rubber particulates are chemically pretreated with sodium hydroxide. The reduced adhesion at the interfacial surface between cement and rubber grains is a critical factor for the reduction of the compressive strength in rubberized concrete. See, Chou, L. H.; Lu, C. K.; Chang, J. R.; Lee, M. T., “Use of waste rubber as concrete additive”, Waste Manage Res 2007, 25 (1), 68-76.
The art is in need of improved techniques for modifying the surface properties of rubber particles in order to enhance the adhesion between the particles and surrounding cement matrix and to manufacture stronger rubberized concrete.